Scalp electrodes of the type under discussion herein are used to record the heart rate of the unborn fetus before and during birth. The electrode is introduced vaginally and fastened to the scalp of the fetus, usually by screwing in a spiral wire. It is understood that this monitoring method--also called "direct ECG"--can only be used after rupture of the membranes. Prior to that, other monitoring methods such as the Doppler ultrasound method have to be used.
The physiological signal derived from the scalp of the fetus is usually fed to an analysis unit that extracts the individual heartbeats (for example the R waves of the ECG) and uses them to determine the heart rate from beat to beat (for example as the reciprocal of the time interval between two heartbeats). The heart rate obtained in this manner is referred to as a "beat-to-beat" heart rate. Its time profile, shape, and reproduced frequency components provide a series of important diagnostic data concerning the fetal condition. This applies particularly if, concurrently with the heart rate, the mother's labor activity is also recorded. The correlation between the two measured variables also provides significant diagnostic information. For example, oxygen deficiency in the fetus due to a loop in the umbilical cord can be detected promptly, and suitable countermeasures (such as Caesarean section) can be taken. Devices or monitors that record fetal beat-to-beat heart rate and maternal labor activity are also known as cardiotocographs or fetal monitors.
Of the various possibilities for deriving the baby's heart rate, direct ECG derived with a fetal scalp electrode supplies by far the best signal, since the transducer is in galvanic contact with the fetal body. When the mother is bearing down this is in fact, according to the present state of the art, the only way to obtain a reliable heart rate indication. Nevertheless, unlike the other methods such as Doppler ultrasound, heart sound measurement, etc., this is an invasive measurement method with all the associated disadvantages. An additional complicating factor is that because of the vaginal introduction, the scalp electrode is difficult to apply.
In the past, various aids had already been developed to facilitate application of the scalp electrode. The typical introduction aid comprises an outer tube in which the electrode head (with the spiral wire or some other fastening means at its proximal end) and a cylindrical inner member, preferably an inner tube, are arranged. The electrode head and the inner tube are connected rotatably fixed (i.e., nonrotatably with respect to each other) but disengageably, for example by means of a bracket extending back from the electrode head into a recess of the inner tube, or a suitable square piece. The outer tube holds these two elements together during the introduction and application process, and protects both the mother and the baby from inadvertent injury due to the spiral wire, since the outer tube projects beyond the wire during the introduction process.
To apply the electrode, the doctor first introduces the outer tube into the birth canal until it touches the fetal scalp. Holding the outer tube with one hand, he then slides the inner tube forward with the other hand until the spiral wire attached to the electrode head touches the baby's head. Then the inner tube is rotated so that the spiral electrode perforates the fetal scalp and penetrates into it. The inner tube can now be withdrawn; this is possible because the electrode head and inner tube are not fastened to one another axially. Then the outer tube is also withdrawn.
In conventional designs the connecting wire of the electrode head is guided through the inner tube; after the two tubes are pulled out, the connecting cable remains in the birth canal and, after application, is connected to a suitable monitoring unit, for example a fetal monitor. Of course it is also possible to provide, instead of a connecting wire, for telemetric transmission or the like, although so far no scalp electrodes based on this principle are available on the market.
Fetal scalp electrodes of the aforesaid type are known in the art and have already been mentioned frequently in the patent literature, for example in U.S. Pat. No. 28,990 or U.S. Pat. No. 4,301,806.
One significant problem with the introduction mechanism for fetal scalp electrodes just described is its complex and unsafe handling. For example, it may happen that the inner tube is not kept in constant engagement with the electrode head. In this case it is no longer possible to transfer torque from the inner tube to the electrode head, such that the electrode cannot be applied. If this happens, the introduction aid must be withdrawn. Although it would theoretically be possible to bring the inner tube and the electrode head back into engagement, this would require grasping the electrode head, meaning that its sterility cannot be guaranteed. If disposable parts are used, they must therefore be discarded. Resterilization is required even in the case of reusable parts.
A feature already known in the art that eliminates the aforementioned problem consists in guiding the electrode cable through the inner tube and clamping it at the distal end of the inner tube. If tension is exerted on the connecting cable, it keeps the electrode head in contact with the inner tube so that the latter cannot unintentionally become detached. According to another solution, also known, a clamp can be placed on the electrode cable, said clamp being applied to the distal end of the inner tube.
Both proposals, however, have the critical disadvantage that the doctor or midwife applying the electrode must not under any circumstances forget to disengage the clamping of the electrode cable after the electrode is applied. If this is forgotten, the electrode is withdrawn together with the inner tube, so that the spiral wire is torn out of the skin of the fetal head. The known devices therefore entail a considerable risk of injury to the baby. There also exists a further handling problem that is not solved by clamping the electrode cable, and that can be described as follows:
During introduction it is necessary to maintain the relative position of the inner and outer tubes. For example, if the inner tube is pushed forward a bit too far, the spiral wire projects out of the introduction aid and may injure the mother or the baby. On the other hand, if the outer tube is inadvertently pushed too far forward, it may happen that the electrode head is driven along with it and disengages, in the manner already mentioned, from the inner tube.
Upon introduction of the electrode, the doctor must therefore use one hand to grasp both the outer tube, and the inner tube projecting from it, at their distal ends, thus establishing the relative position of the two tubes. With the same hand he must also advance the introduction aid; the second hand is needed to guide the outer tube. It is evident that this type of handling is extremely inconvenient.
One proposal, known from the prior art, for solving this second problem consists in mutual locking of the inner and outer tubes, for example by means of a snap lock. However, with these introduction aids (also known) the connector cable for the electrode must be manually retained in order to prevent the electrode head from disengaging from the inner tube.
It would be possible to combine the two features discussed above--clamping the electrode cable and mutually snap-locking the inner and outer tube although this attempt has not yet been made in practice. It is evident, however, that even this combination would not completely solve the underlying problems, since in order to prevent injury to the fetus one must under no circumstances forget to disengage the electrode cable clamp. This problem is fundamentally bound up with the use of a clamping mechanism, and cannot be eliminated even by combining the two features described above. In addition, the mechanical features required to snap-lock the two tubes and clamp the electrode cable are very complex, and increase the manufacturing costs of the electrode.